Does the intermediate junction box of the instrument need to be grounded?
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As the title suggests, let’s discuss whether the junction box in the instrument panel needs to be grounded If necessary, is grounding done by connecting to the flat steel grounding of electrical equipment, buildings, or pipe rack columns?
This post was last edited by waneng on 2017-6-27 at 15:59. The junction box in the instrument panel must be safely grounded; for this purpose, a yellow-green grounding wire is used to connect to a metal conductor that is already connected to the ground, such as steel structures, cable trays, or conduit systems. The instrument shielding ground is achieved by connecting the branch cables to the separate screens of the multi-core cable through terminals; the main screen of the multi-core cable is usually left unconnected on the junction box side, with the working ground being connected uniformly in the control room.
On the rooftop. Additionally, to build on what was said above: a multi-core cable refers to the main cable that runs from the intermediate junction box to the cabinet, while branch cables are those that run from the intermediate junction box to the field meters. One thing to rule out is the scenario where, in some design schemes, the shielding layers of the main cable and the branch cables are connected together at the junction box; in such a case, it’s as though there is only one shielding layer. In this situation, the shielding layer of the cable only needs to be connected indirectly at the cabinet, with the end portion remaining unconnected on the side of the meter. The shielding layer of the main cable is required to be grounded on one side, that is, it should be grounded uniformly within the cabinet area; therefore, the side of the junction box remains ungrounded. Additionally, it is preferable that both the main shield and the individual shields of the main cable be grounded uniformly within the cabinet area, which is different from what happens upstairs ； The shielding layer of the branch cable is left suspended at the meter head, while the junction box is grounded.
The regulations state that grounding is not required for voltages below 36V. The specification also requires that the shielding be single-ended grounded on the control room side (working ground), ensuring reliability and stability. Using grounding connected to the steel structure or protective pipes are both non-standard methods of grounding; their reliability cannot be guaranteed, and they are merely used to meet the inspection requirements of some uninformed people. Strictly in accordance with the requirements, they should all be connected from a dedicated electrical grounding busbar, or through a dedicated line from the electrical grounding network.
Consider only shielding: If the DCS side requires multi-point grounding, then grounding copper bars need to be installed, and the grounding network must be established in coordination with the electrical team. If single-point grounding is implemented in accordance with the specifications, there is no need to provide a grounding connection; instead, special terminals must be used to maintain the continuity of the shielding layer. If there are low-voltage cables in the junction box, protective grounding must be considered.